As an imaging apparatus with a solid state imaging device, such as CCD or CMOS, digital cameras have been widely used. The solid state imaging device is provided with a plurality of pixels that store or accumulate electric charges in accordance with the quantity of light incident on the individual pixel. The charges stored in the respective pixels are taken as analog image signals. Because the image signals have a low signal level at the output of the imaging device, they are amplified by an amplifier circuit in the digital camera. The amplified image signals are digitalized, and subjected to image processing, such as a gamma-correction, white-balance correction and an image quality correction. The processed digital image signals are written on a storage medium like a memory card.
The digital cameras have a disadvantage that their dynamic range is not sufficiently wide. This is because the pixels of the imaging device cannot store the electric charges any more after the stored amount reaches a saturation level, as shown in FIG. 12. As a result, so far as the subject brightness is so low that the charge storage amount is less than the saturation level, the charge storage amount corresponds to the subject brightness, so that the obtained image signal has a level corresponding to the subject brightness, enabling reproducing the tone in the image. But when the subject brightness is so high that the charge stored in the pixel reaches the saturation level, the pixel cannot store the charges any more and the image signal values of the saturated pixels stop at the maximum even through the subject brightness is higher. Then, corresponding image pixels look white and do not reproduce gradations in highlight portions of the subject, which is so-called clipped highlight.
To solve this problem, an imaging apparatus has been suggested for example in Japanese Laid-open Patent Application No. 2003-218343, that is provided with high sensitive main pixels and low sensitive sub pixels to widen the dynamic range by composing image signals from the main pixels and the sub pixels. As the main pixels are high sensitive, image signals from the main pixels have large values in a shorter exposure time, so they are suitable for reproducing gradations in a low brightness range. On the contrary, as the sub pixels are low sensitive, they would not instantly be saturated with charges even while the subject brightness is high, so they are suitable for reproducing gradation in a high brightness range. Therefore, composing the image signals from the main and sub pixels achieves a wider dynamic range.
Because the main pixels have a higher sensitivity than the sub pixels, image signals from the main pixels, hereinafter called the main pixel image signals, have larger signal values or a higher signal level than image signals from the sub pixels, hereinafter called the sub pixel image signals, when they are taken under the same conditions.
Since the prior imaging apparatus has only a signal amplifier, the main pixel image signals and the sub pixel image signals are amplified at the same gain, the rate of amplification. As a result, the sub pixel image signals are not sufficiently amplified while the gain is set at a value for amplifying the main pixel image signals to proper levels. If the gain is set at a value for amplifying the sub pixel image signals to proper levels, the main pixel image signals are excessively amplified.